I thought the exact same thing, the first time I destroyed it (I've done it a few times for the different questlines). The way I see it there are two possible reasons: A, Obsidian "forgot" to implement it. B, if they had gone out you would have BoS scribes, knights, and paladins all over the place.
haha....
"So....tacos?"
Yeah it would be a bit odd, still if you managed to escape detection they should run out, it would be funny to see them, maybe they make a steady pilgrimage around the wasteland looking for places to settle, slowly getting whittled down or leaving and integrating into normal life like the Enclave.
It just feels weird that they just...let themselves die, so happily.